Recently, in consideration of environmental issues, vehicles using electric motors as driving sources, such as electric vehicles, hybrid vehicles and fuel cell vehicles are attracting attention. Such a vehicle is provided with a rechargeable power storage unit for supplying power to the electric motor and for converting kinetic energy to electric energy and storing the same at the time of regenerative braking.
In the vehicle having an electric motor as the driving source as described above, power supply of larger capacity is desirable in order to improve running performance such as acceleration characteristic and mileage per charge. As a method of increasing power supply capacity, provision of a plurality of power storage units has been proposed.
By way of example, U.S. Pat. No. 6,608,396 discloses an electrical motor power management system providing a desired high DC voltage level in a high-voltage vehicle traction system. The electrical motor power managements system includes: a plurality of power stages for providing DC power to at least one inverter, each stage including a battery and boost/buck DC-DC converter, and wired in parallel; and a controller controlling the plurality of power stages such that the batteries of the plurality of power stages are uniformly charged/discharged whereby the battery voltage to the at least one inverter is maintained by the plurality of power stages.
On the other hand, the power storage unit implemented, for example, by a chemical battery stores electric energy utilizing electro-chemical reaction and, therefore, its charge/discharge characteristic is much susceptible to the influence of temperature. Generally, the charge/discharge characteristic tends to lower at a lower temperature, while deterioration proceeds faster at a higher temperature. Therefore, it is necessary to control temperature of the power storage unit mounted on a vehicle such that it is kept within a prescribed temperature range. Therefore, a power storage unit mounted on a vehicle is often prepared as a package including a cooling fan.
When a plurality of power storage units are to be mounted on a vehicle, however, the package structure may lead to variation in cooling performance among the power storage units. As a result, even if the power storage units are made to have the same rated value and charge/discharge power with each other, temperature may still vary among the power storage units. Specifically, cooling of one electric storage unit may be relatively too much and the unit temperature may decrease, or cooling of one electric storage unit may be relatively insufficient and the unit temperature may increase.
The electrical motor power management system disclosed in U.S. Pat. No. 6,608,396 does not at all consider the temperature of batteries (power storage units) and, therefore, temperature variation among the plurality of power storage units cannot be avoided.